The present invention relates generally to filtering signals in microwave propagating media and, more particularly, to a method and apparatus for a low loss high-pass microwave filter for microstrip circuits.
Microstrip circuits are made up of a substrate formed of a dielectric material which separates a ground plane on one side of the substrate from a signal carrying microstrip line on the other. Use of microstrip circuits is preferred in many applications because their cost is negligible when compared to costlier waveguide structures. Additionally, electrical components are easily coupled to microstrip circuits, which have wide band and low loss characteristics. Microstrip circuits are also light in weight and easily printed using conventional printed circuit board technology.
A problem with using microstrip circuits is the design of a low loss high-pass filter which can be included on the microstrip circuit yet occupy a minimum amount of space on the circuit. A number of approaches to the solution of this problem are known in the prior art including edge-coupled lines, impedance transformers and low frequency signal rejection techniques. Waveguides offer another solution to the problem of low loss high-pass filters because of the inherent high attenuation of signals below the cutoff frequencies of waveguides. In addition, there are substantial low loss frequency ranges above the cutoff frequencies of waveguides. Unfortunately, the characteristics of waveguides are not realizable directly on microstrip circuits because the dominant desired mode in a microstrip circuit or quasi-transverse-electric-magnetic (quasi-TEM) mode has no cutoff frequency and will pass all frequencies down to DC. However, waveguide structure can be combined with microstrip circuits to provide high-pass filters.
For example, rectangular waveguides have been considered for use with microstrip circuits to provide high-pass filters as illustrated by an article entitled "About the Frequency-Dependent Characteristics of a Microstrip-Waveguide Transition" published in Arch. Elektron and Uebertragungstech., vol. 35, no. 2, pages 69-71, February, 1981 by Gunter Kompa. Unfortunately, such high-pass filters are generally restricted to a 2:1 usable frequency ratio in order to avoid losses due to the production of undesirable higher-order modes. Further, transitions of the type considered by Kompa are lossy and lead to external surface modes which allow certain lower-frequency modes to propagate on the outer surface of the waveguide.
Accordingly, there is a need for a low loss high-pass microwave filter which can be readily utilized with microstrip circuits and will occupy little space on the microstrip circuit.